The present invention relates to an electric load drive apparatus for supplying a trapezoidal wave current to an electric load.
An electric load, such as a lamp or a coil, has an impedance (i.e., a resistance value) varying in response to heat generation during its activation. In the beginning of activation, the electric load has a relatively small resistance value. A relatively large current will flow across the electric load. Accordingly, the electric load will cause significant noise. To solve this problem, U.S. Pat. No. 6,184,663 corresponding to the unexamined Japanese patent publication 2000-138570 discloses an electric load drive apparatus shown in FIG. 5.
A conventional electric load drive circuit 1 shown in FIG. 5 comprises a resistor 4 and a MOS transistor 5 serially interposed between a battery 2 and an electric load 3 (such as a lamp), a trapezoidal wave generating circuit 6 which generates a trapezoidal wave signal Sb in accordance with a drive command signal Sa, and a current control circuit 7 which compares the trapezoidal wave signal Sb with a voltage value detected by the resistor 4 to control a gate voltage of MOS transistor 5.
Although not shown in the drawing, the trapezoidal wave generating circuit 6 consists of a capacitor, a charging constant-current circuit, and a discharging constant-current circuit. A trapezoidal wave signal Sb is produced between both terminals of the capacitor of trapezoidal wave generating circuit 6. The upper edge voltage of trapezoidal wave signal Sb is controlled to be a constant value. The current control circuit 7 consists of a voltage conversion circuit 8 and an error amplification circuit 9. The voltage conversion circuit 8 produces a trapezoidal wave signal Sd by inverting the trapezoidal wave signal Sb. The trapezoidal wave signal Sb has an electric potential defined with respect to a ground potential, while the trapezoidal wave signal Sd has an electric potential defined with respect to the reference battery voltage VB. The error amplification circuit 9 compares the inverted trapezoidal wave signal Sd with the voltage applied to the resistor 4. The error amplification circuit 9 controls the gate potential of MOS transistor 5 so as to equalize the voltage applied to the resistor 4 with the inverted trapezoidal wave signal Sd.
According to this arrangement, a lamp current (i.e., the current flowing across the electric load 3 (=lamp)) linearly increases in accordance with voltage increase of trapezoidal wave signal Sb in the beginning of activation of this lamp. The lamp current linearly decreases in accordance with voltage decrease of trapezoidal wave signal Sb after deactivating the lamp. When the drive command signal Sa is a cyclic pulse signal, the brightness of lamp can be adjusted by changing the duty ratio of drive command signal Sa.
When the electric load 3 is a lamp installed in an automotive vehicle, it is not assured that the lamps exchanged by a user or a car repairer are always the same type. For example, there is the possibility that a user may install another type of lamp which has a different rated current (i.e., impedance).
According to the above-described conventional electric load drive circuit 1, the upper edge of trapezoidal wave signal Sb is determined in the following manner. It is now assumed that the installed electric load 3 is a specific lamp having the largest rated current (e.g., rated current=6A). Under this condition, the upper edge of trapezoidal wave signal Sb is determined so that MOS transistor 5 operates in a saturation region until the trapezoidal wave signal Sb reaches the upper edge voltage and MOS transistor 5 operates in a linear region after the trapezoidal wave signal Sb has reached the upper edge voltage.
By determining the upper edge voltage in this manner, it becomes possible to suppress drain loss of MOS transistor 5.
However, if an installed lamp has a rated current having a rated current smaller (i.e., an impedance larger) than that of the above-described specific lamp, brightness adjustment of a lamp may cause a trouble as explained hereinafter with reference to FIG. 6.
FIG. 6 is a time chart showing the waveforms of various portions in the conventional electric load drive circuit in the case an electric load is a specific lamp having a rated current of 6A or another type of lamp having a rated current of 6A. The waveforms shown in FIG. 5 are (a) drive command signal Sa, (b) trapezoidal wave signal Sb, (c) lamp current (rated current=6A), (d) lamp voltage (rated current=6A), (e) lamp current (rated current=3A), and (f) lamp voltage (rated current=3A).
When a lamp having a rated current of 6A is connected to the electric load drive circuit 1, the lamp voltage (i.e., a voltage applied to the lamp) increases or decreases in accordance with ascent or descent of trapezoidal wave signal Sb. The trapezoidal wave signal Sb is derived from drive command signal Sa. The lamp voltage is equal to the battery voltage VB during a term the trapezoidal wave signal Sb is equal to the upper edge voltage. For example, a current supply threshold can be set to a mid point (3A) of the current amplitude with respect to a trapezoidal wave lamp current (as shown by an alternate long and two short dashes line in FIG. 6(e)). The duty ratio of lamp current, being set based on the current supply threshold, can be always equalized with the duty ratio of drive command signal Sa. It becomes possible to adjust the brightness of lamp according to a given command.
On the other hand, when a lamp having a rated current of 3A is connected to the electric load drive circuit 1, the lamp current increases in accordance with ascent of trapezoidal wave signal Sb. However, the lamp current becomes equal with the battery voltage VB at time tb and stops increasing before the trapezoidal wave signal Sb reaches the upper side voltage. Furthermore, the trapezoidal wave signal Sb starts decreasing at time td. Then, waiting for passage of a time lag, the lamp voltage starts decreasing at time te. As a result, significant deviation is caused between the waveform of the lamp current and the waveform of trapezoidal wave signal Sb at each of time durations tb-tc and td-te.
Hence, the above-described duty ratio of lamp current (i.e., effective current supply time) becomes larger than the duty ratio of drive command signal Sa (i.e., commanded current supply time).
In this manner, when a lamp having a rated current smaller than that of the above-described specific lamp is connected to the electric load drive circuit 1, the duty ratio of lamp current (i.e., effective current supply time for determining the brightness of lamp) cannot agree with the light adjusting command given by drive command signal Sa. Furthermore, the duty ratio of lamp current cannot be lowered sufficiently. The brightness of lamp may not be reduced sufficiently.
In view of the foregoing problems of the prior art, the present invention has an object to provide an electric load drive apparatus which is capable of equalizing an actual current supply time with a commanded current supply time designated by a drive command signal irrespective of impedance of this electric load.
In order to accomplish the above and other related objects, the present invention provides an electric load drive apparatus comprising a switching element provided in an electric power supply path extending from a direct-current power source to an electric load. A detecting resistor, connected in series with the switching element, detects a voltage value representing a load current flowing across the electric load via the switching element. A saturation state detecting section is provided for outputting a current saturation signal during a term an upper limit current flows across the electric load. The upper limit current is a maximum current flowable via the switching element. A signal generating section is provided for producing a trapezoidal wave signal. The voltage value of this trapezoidal wave signal starts changing in accordance with increase of the load current when a drive start command signal is entered, stops changing in response to the output of the current saturation signal generated from the saturation state detecting section, and then starts changing in accordance with decrease of the load current when a drive stop command signal is entered. And, a current control section is provided for comparing the trapezoidal wave signal produced from the signal generating section with the voltage value detected by the detecting resistor and then controlling the switching element based on the trapezoidal wave signal in such a manner a trapezoidal wave current flows across the electric load.
According to the arrangement of the electric load drive apparatus of this invention, the detecting resistor produces a voltage proportional to the load current flowing across the electric load. The current control section compares the trapezoidal wave signal produced from the signal generating section with the voltage value detected by the detecting resistor and then controls the switching element based on the trapezoidal wave signal. Hence, a trapezoidal wave current flows across the electric load. As a result, it becomes possible to suppress the load current from abruptly increasing in the beginning of activation of the electric load. Thus, it becomes possible to suppress generation of noise which may be produced due to abrupt change of load current.
The load current flowing across the electric load via the switching element from the direct-current power source has an upper limit value which is dependent on voltage of direct-current power source, impedance of the electric load, and open/close state of the switching element. The saturation state detecting section detects a current saturation state that the upper limit current flows across the electric load via the switching element in the open/close controllable range of the switching element. The signal generating section causes the voltage value of the trapezoidal wave signal to start changing in accordance with increase of the load current when the drive start command signal is entered. And then, the signal generating section causes the voltage value of the trapezoidal wave signal to stop changing in response to detection of the current saturation state.
Namely, according to the signal generating section of this invention, the upper edge voltage of the trapezoidal wave signal is not controlled to a constant value. Rather, the upper edge voltage of the trapezoidal wave signal is controlled to a value corresponding to the above-described upper limit current determined by the voltage of the direct-current power source and the impedance of the electric load. Hence, it becomes possible to prevent the trapezoidal wave signal from generating a command exceeding the above-described upper limit current which is not actually obtained. The electric load can always receive a current corresponding to the trapezoidal wave signal. As a result, when the drive stop command is entered and the trapezoidal wave signal starts changing in accordance with decrease of the load current, the load current immediately starts decreasing in accordance with the trapezoidal wave signal.
Accordingly, irrespective of impedance of an electric load to be connected, the load current always follows the trapezoidal wave signal produced based on the drive command signal. The effective current supply time of a trapezoidal wave load current can be always equalized with the commanded current supply time designated by the drive command signal.
Furthermore, it is preferable that the switching element is constituted by a transistor capable of controlling the load current in accordance with an input voltage entered to a control terminal thereof, the current control section comprises an operational amplifier outputting a voltage corresponding to a potential difference between the trapezoidal wave signal produced from the signal generating section and the voltage value detected by the detecting resistor, and the current control section controls the input voltage supplied to the control terminal of the transistor in accordance with the output voltage of the operational amplifier.
According to this arrangement, the error amplification function of operational amplifier allows the transistor to control its ON state in such a manner that the trapezoidal wave signal of the signal generating section with the voltage value detected by the detecting resistor. The electric load always receives the current corresponding to the commanded trapezoidal wave signal.
Furthermore, it is preferable that the saturation state detecting section comprises a comparator comparing a control voltage of the transistor with a predetermined reference voltage.
According to this arrangement, if the trapezoidal wave signal exceeds a value corresponding to the upper limit current flowable across the above-described electric load, a deviation between the load current and the trapezoidal wave signal will increase. The control voltage of the transistor will abruptly increase due to feedback control by the operational amplifier. The comparator constituting the saturation state detecting section compares the control voltage of the transistor with the predetermined reference voltage, thereby detecting such an abrupt voltage increase (i.e., current saturation state). Therefore, the saturation state detecting section of the present invention can surely detect the current saturation state even if the voltage of direct-current power source fluctuates.
Furthermore, it is preferable that the signal generating section comprises a capacitor outputting the trapezoidal wave signal, a first constant-current circuit connected in series with the capacitor, a second constant-current circuit connected in parallel with the capacitor to perform a constant-current operation during a term the drive stop command signal is entered, and a third constant-current circuit connected in series with the first constant-current circuit to input an output current of the first constant-current circuit during a term the saturation state detecting section outputs the current saturation signal.
According to this arrangement, both of the second constant-current circuit and the third constant-current circuit suspend their constant current operations when the drive start command signal is entered under a condition the drive stop command signal and the current saturation signal are not entered into the signal generating section. Hence, a constant charging current flows into the capacitor from the first constant-current circuit. The voltage applied to the capacitor starts changing in accordance with increase and decrease of load current. Subsequently, when the current saturation signal is entered, the third constant-current circuit receives the output current of the first constant-current circuit. The charging of the capacitor is interrupted. The voltage applied to the capacitor stops changing. Subsequently, when the drive stop command signal is entered, the discharging current flows into the second constant-current circuit from the capacitor. The voltage applied to the capacitor starts changing in accordance with increase and decrease of load current. As a result, the above-described trapezoidal wave signal is produced from both terminals of the capacitor.
Furthermore, it is preferable that the signal generating section comprises a limit circuit for suppressing a voltage applied to the capacitor within a predetermined voltage.
According to this arrangement, the upper edge voltage of the trapezoidal wave signal is limited to the predetermined value. This surely prevents the load current from exceeding the above-described predetermined value even when the voltage of direct-current power source is excessively large or when the electric load has an excessively small impedance.
Moreover, it is preferable that each of the drive start command signal and the drive stop command signal is a PWM signal having a duty ratio corresponding to the load current. As the command current supply time commanded by the drive command signal is equalized with the effective current supply time of the load current, the current having a duty ratio identical with the duty ratio of the PWM signal flows across the electric load.
Furthermore, the present invention has an object to provide a control apparatus for a power MOS transistor which is capable of suppressing noise and heat generation.
In order to accomplish the above and other related objects, the present invention provides a control apparatus for a power MOS transistor comprising a power MOS transistor interposed between an electric load and a power source for causing a trapezoidal pulse current to flow across the electric load when a control voltage is applied to a gate terminal of the power MOS transistor. A gate-source voltage detecting section is provided for detecting a gate-source voltage of the power MOS transistor when the control voltage is applied to the gate terminal of the power MOS transistor. A feedback section is provided for obtaining deviations of rise time and fall time of a present load current waveform with respect to their target values based on the gate-source voltage of the power MOS transistor detected by the gate-source voltage detecting section. The feedback section performs a feedback control of the power MOS transistor so as to eliminate the deviations of rise time and fall time.
In this manner, even if the resistance value of an electric load changes due to activation of the electric load, it becomes possible to suppress the rising and falling gradients of the load current within predetermined values. It is effective to suppress noise and heat generation.
Furthermore, it is preferable that the rise time and the fall time of the present load current waveform are obtained by measuring the duration of a saturation region in the transistor characteristics.
It is also preferable that control apparatus for a power MOS transistor of the present invention further comprises a first comparing section for comparing the detected gate-source voltage with a first judgement value being set between an OFF voltage and a threshold voltage. A second comparing section is provided for comparing the detected gate-source voltage with a second judgement value being set between an ON voltage and the threshold voltage. And, a time measuring section is provided for measuring elapse of time between the first judgement value and the second judgement value based on comparing results obtained by the first comparing section and the second comparing section. The time measuring section designates the measured time elapse as the duration of the saturation region in the transistor characteristics.
It is also preferable that the feedback control of the power MOS transistor is performed in such a manner that the rise time and the fall time become constant.
It is also preferable that the electric load is a lamp.
Furthermore, the present invention has an object to provide an electric load drive apparatus which is capable of suppressing noise caused due to supply or stop of electric current in a case a trapezoidal current is supplied to an electric load.
In order to accomplish the above and other related objects, the present invention provides an electric load drive apparatus comprising a signal generating section for generating a current command signal in accordance with a drive command signal entered from an external device. The current command signal has a trapezoidal waveform increasing from a first level to a second level in response to a drive start command and decreases from the second level to the first level in response to a drive stop command. A current drive section, provided in a current supply path extending from a direct-current power source to an electric load, detects a load current flowing across the electric load and outputs a trapezoidal current to the electric load based on a comparison between the detected load current and the current command signal. A measuring section is provided for measuring a rise time of the load current corresponding to increase of the current command signal and a fall time of the load current corresponding to decrease of the current command signal. And, a gradient control section is provided for controlling a change rate of the current command signal produced from the signal generating section in such a manner that the measured rise time is equalized with a predetermined reference rise time and the measured fall time is equalized with a predetermined reference fall time.
According to this arrangement, the current drive section detects the load current flowing across the electric load and outputs the trapezoidal current to the electric load based on a comparison between the detected load current and the current command signal. With this arrangement, it becomes possible to limit the change rate of the load current in a moment the current supply to the electric load is started or stopped. It becomes possible to effectively reduce the noise caused by the load current changing abruptly. Furthermore, as the load current is directly controlled, distortion of current can be suppressed to a small value and the noise can be surely reduced.
In this case, for example, the reference rise time and the reference fall time are determined so as to suppress the generated noise and the drive loss to an allowable level. The gradient control section controls the change rate (i.e., increase rate and decrease rate) of the current command signal in such a manner that the measured rise time is equalized with the reference rise time and the measured fall time is equalized with the reference fall time. As a result, the load current increases for the reference rise time and decreases for the reference fall time irrespective of impedance of electric load. Namely, the rise and fall of load current are limited to minimum change rates in an allowable region considering the drain loss of current drive section. Thus, it becomes possible to reduce the noise generated when the electric load is activated or deactivated.
It is preferable that the gradient control section comprises a gradient limit section for preventing the change rate (i.e., increase rate and decrease rate) of the current command signal from exceeding a predetermined regulation value. This effectively prevents the generated noise from exceeding an allowable level when the connected electric load has a small impedance.
Furthermore, it is preferable that the measuring section comprises a voltage detecting section for detecting a load voltage applied on the electric load when the current command signal is in the second level, a reference voltage generating section for generating a first reference voltage and a second reference voltage having mutually different ratios with respect to the detected load voltage of the voltage detecting section, and a time measuring section for measuring a time required for the load voltage applied on the electric load to vary from the first reference voltage to the second reference voltage or vice versa.
According to this arrangement, the rise and fall times of the load voltage are measured to indirectly detect rise and fall times of the load current. When the current command signal is in the second level (i.e., the upper edge of a trapezoidal wave), the load current is at the upper edge of a trapezoidal wave. Hence, by measuring the time required for the load voltage to vary from the first reference voltage to the second reference voltage or vice versa, it becomes possible to obtain the rise time and the fall time of the load current.
Furthermore, it is preferable that the signal generating section comprises a capacitor for outputting the current command signal, a first current output circuit having a controllable output current for supplying a charge current of the capacitor, and a second current output circuit having a controllable output current for supplying a discharge current of the capacitor.
According to this arrangement, when the drive start command is given, the first current output circuit supplies charge current to the capacitor to increase the capacitor from the first level to the second level. On the other hand, when the drive stop command is given, the discharge current flows from the second current output circuit to allow the voltage of capacitor to decrease from the second level to the first level.
Furthermore, it is preferable that the gradient control section comprises a first current control section for controlling the output current of the first current output circuit based on a difference between the rise time measured by the measuring section and the reference rise time, and a second current control section for controlling the output current of the second current output circuit based on a difference between the fall time measured by the measuring section and the reference fall time. With this arrangement, it becomes possible to independently control the rise time and the fall time of the load current.
Furthermore, it is preferable that each of the first and second current output circuits produces a current in accordance with an entered control voltage, and each of the first and second current control sections comprises a capacitor for outputting the control voltage, a charge circuit for charging the capacitor when the difference is positive, and a discharge circuit for discharging the capacitor when the difference is negative.
According to this arrangement, the first and second current control sections of the gradient control section are constituted as a charge/discharge circuit consisting of the capacitor, the charge circuit, and the discharge circuit. The output voltages of these charge/discharge circuits are used to control the output currents of the first and second current output circuits of the signal generating section. When the rise time of the load current is longer than the reference rise time (i.e., when the difference is positive), the capacitor is charged to increase the output current of the first current output circuit. As a result, the rise time of the current command signal can be reduced. The same explanation is applied to the fall of load current.
Furthermore, it is preferable that the current drive section comprises a switching element provided in a current supply path extending from the direct-current power source to the electric load, a current detecting section for detecting the load current and producing a current detecting signal, and a load current control section for controlling the switching element based on a difference between the current detecting signal and the current command signal.
According to this arrangement, the load current control section controls on-off condition of the switching element provided in the current supply path extending from the direct-current power source to the electric load in such a manner that the difference between the current detecting signal and the current command signal can be reduced. With this arrangement, the current drive section can supply a trapezoidal current to the electric load in accordance with the current command signal.
Moreover, it is preferable that the drive command signal is a cyclic pulse signal (e.g., PWM signal) having a controlled duty ratio. In this case, in each period of the pulse signal, the measuring section measures the rise time and the fall time of the load current while the gradient control section controls the increase rate and the decrease rate of the current command signal. Hence, it becomes possible to always and accurately equalize the rise time and the fall time of the load current with their reference values. The noise can be surely reduced.
Furthermore, the present invention has an object to provide an electric load drive apparatus capable of suppressing distortion of waveform appearing at a decreasing start time of load current in a case a trapezoidal current is supplied to an electric load.
In order to accomplish the above and other related objects, the present invention provides an electric load drive apparatus comprising a transistor provided in a current path extending from a direct-current power source to an electric load for controlling a load current flowing across the electric load based on a control signal. A current detecting section is provided for detecting the load current flowing across the electric load. A signal generating section is provided for generating a current command signal in accordance with a drive command signal entered from an external device. The current command signal has a trapezoidal waveform increasing from a first level to a second level in response to a drive start command and decreasing from the second level to the first level in response to a drive stop command. A current control section is provided for supplying the control signal based on a comparison between a current detection signal detected by the current detecting section and the current command signal in such a manner that a trapezoidal current flows across the electric load in accordance with the current command signal. And, a control voltage limit section is provided for limiting the control signal to a decreasing start level in response to the drive stop command. The decreasing start level is slightly higher than a threshold level of the transistor required to reduce the current flowing across the electric load.
According to this arrangement, the load current is controlled to have a trapezoidal waveform. Thus, it becomes possible to adequately suppress the change rate of load current at a current start or stop time. Thus, it becomes possible to suppress the noise generating due to abrupt change of load current.
In this case, the first level and the second level are determined beforehand so as to correspond to the lower edge and the upper edge of the trapezoidal current command signal. When the impedance of the electric load becomes high, the load current cannot increase up to a current value commanded by the second level of the current command signal due to shortage of direct-current power source voltage. In such a non-following state of load current in the vicinity of the second level, the current detection signal largely deviates from the current command signal. The control signal produced from the current control section becomes large compared with the value adjacent to a threshold value in a case where the transistor actually increases or decreases the current flowing across the current. As a result, excessive electric charge is stored in a capacitor of the transistor (such as a gate capacitor of MOS transistor).
However, the control voltage limit section of the present invention limits the control signal to the decreasing start level in response to the drive stop command. The decreasing start level is slightly higher than the threshold level of the transistor. Thus, it becomes possible to promptly draw the excessive electric charge stored in the capacitor of the transistor (e.g., gate capacitor of MOS transistor). The load current quickly follows the reduction of control signal corresponding to the reduction of current command signal. Hence, it becomes possible to prevent the load current from decreasing abruptly. As a result, it becomes possible to effectively suppress the noise.
It is preferable that the control voltage limit section limits the control signal to the decreasing start level defined with respect to a source or emitter potential of the transistor in response to the drive stop command.
According to this arrangement, the decreasing start level of control signal is set with respect to the source or emitter potential of the transistor. According to this setting, no adverse influence is given from the load voltage applied to the electric load. Thus, irrespective of direct-current power source voltage, it becomes possible to accurately limit the control signal to the decreasing start level adjacent to the threshold level.
Furthermore, it is preferable that the current control section comprises an error amplifying section for outputting the control signal corresponding to a difference between the current detection signal and the current command signal, and the control voltage limit section is connected between an output terminal of the error amplifying section and a source or emitter terminal of the transistor.
According to this arrangement, the error amplifying section outputs the control signal corresponding to a difference between the current detection signal and the current command signal. The control voltage limit section forcibly limits the voltage (i.e., control voltage) of the output terminal of the error amplifying section to the decreasing start level in response to the drive stop command. Thus, it becomes possible to accomplish the current control the transistor within a short time.
Furthermore, it is preferable that the control voltage limit section is connected between gate and source terminals of the transistor or between base and emitter terminals of the transistor.
According to this arrangement, the control voltage limit section limits the gate-source voltage or base-emitter voltage of the transistor to the decreasing start level in response to the drive stop command.
Furthermore, it is preferable that the control voltage limit section is constituted by a serial circuit of a switch circuit turning on in response to the drive stop command and a voltage limit circuit having a clamp voltage equivalent to the decreasing start level.
According to this arrangement, the switch circuit constituted by a transistor is turned on in response to the drive stop command. In this case, the control signal is limited to the clamp voltage by the voltage limit circuit. The voltage limit circuit can be constituted by a Zener diode.